Wavelength-division-multiplexed (WDM) optical transmission systems carry multiple wavelength channels simultaneously on a single guiding optical line. Integrated optical circuits, comprising various functional optical components, employ waveguiding structures that can couple light efficiently to and from optical fibers, and offer the possibility of WDM signal processing, such as switching and routing, on a chip in the optical domain. Dynamic reconfiguration of functional optical components that operate on a subset of the used WDM spectrum may be employed to reroute one or more WDM signals around a broken link in the network, to add/drop one or more wavelength channels on a WDM fiber at a network node, or to perform other signal processing operations on a wavelength-selective basis. A device that performs such functions is referred to as a reconfigurable optical add-drop multiplexer (R-OADM), and employs channel add-drop filters.
Optical channel add-drop filters are devices that typically have an input port, at least a drop or add port, a through port, and preferably a further port which, in combination with the drop or add port, forms a pair of add and drop ports, and support narrow passbands covering typically a single wavelength channel. They enable transmission of a signal in the selected wavelength channel within the passband from the input port to the drop port with low loss (preferably less than 3 dB), while suppressing crosstalk from signals in other wavelength channels in the operating wavelength range (OWR) of the filter (preferably by at least 30 dB). All channels outside the filter passband and in the OWR of the filter are transmitted from the input port to the through (or express) port, preferably with much less than 3 dB insertion loss. The selected wavelength channel within the filter passband is preferably fully removed from the input spectrum with preferably over 30-40 dB extinction of the signal remaining in the through port. This high extinction is required to prevent crosstalk with a new signal, incident at the add port, that is inserted into the through port at the selected channel wavelength. Wavelength channel passbands are typically 10-100 GHz wide and are typically spaced by 25-200 GHz as, for example, specified by the International Telecommunications Union (ITU) wavelength grid standards. For 40 GHz wide filters, typically no more than about 20 ps/nm of dispersion is preferably added by the filter to the channels in both the drop port and the through port. In the through port, much less dispersion is preferable because wavelength channels on a ring network may traverse many R-OADMs via the through port before reaching their destination and being dropped. To support cascading often, i.e., at least 5-20 times, lower dispersion values are desirable. Furthermore, it is preferable that any insertion loss and/or dispersion that is introduced by the filter to express channels, i.e., those passing to the through port, be balanced, i.e., as equal as possible among all of the channels.
Preferably, during the dynamic reconfiguration of optical components such as R-OADMs, i.e., of their add-drop filters, that operate on a subset of the WDM spectrum, the data flow on other express wavelength channels in the through port is not interrupted or deteriorated (e.g., by insertion loss or dispersion) during the reconfiguration operation. This is referred to as hitless switching or hitless reconfiguration of the optical component.
It is further desirable that an optical channel add-drop filter be able to process any single WDM channel within its OWR. The OWR of the filter is preferably a wide optical band, e.g., the C-band communication window of 1530-1570 nanometers (nm). It is desired a filter with only one active passband over the operating wavelength range, and thus, for resonant filters, only one resonance within the optical band, i.e., a spacing between adjacent resonances, or free spectral range (FSR), larger than the OWR. A filter whose operating channel wavelength may be dynamically adjusted is referred to as tunable. A wide tuning range for the center wavelength of the filter passband—a tuning range that covers the OWR—is required to enable to access any channel in that range. Finally, the filter reconfiguration process from dropping one wavelength channel to dropping another wavelength channel or a complete off state (not dropping any channels), is preferably hitless, i.e., transparent to the other WDM channels as described.
Integrated optical filters with a single passband over a wide operating wavelength range can be made using optical resonators, for example microring resonators, with a large FSR equal to or larger than the OWR, such that only one resonance lies within the range. Large FSR resonators can be made by making the resonator small in size so that, in traveling-wave resonators like rings, the path length is short and spaces longitudinal resonances further apart spectrally. Small ring resonators have tight bend radii and optical radiation confined and propagating in such a ring tends to experience bending radiation loss, giving rise to a low quality factor, Q. Radiation loss can be reduced to an acceptable level by designing waveguides using high refractive index contrast (HIC) between the waveguide core and cladding, such as SiN (n˜2 near 1550 nm wavelength) or Si (n˜3.5) core, and silica (n˜1.45) or air (n˜1) cladding. In turn, high-index-contrast resonators are small and require fine lithography, can have significant propagation losses due to surface roughness, and their resonant frequency can be very sensitive to small dimensional errors resulting in fabrication. Furthermore, for very large FSRs desired in some applications, it may not be possible to use a sufficiently high index contrast due to lack of practical materials with a high enough index over the wavelength range of interest. Therefore, methods to extend the usable FSR of a filter are desirable.
In the Vernier scheme, two resonators with different FSRs are cascaded such that a passband is obtained only at those wavelengths where both resonators have a resonance. This enables the use of ring resonators, for example, of larger radii and lower index contrast to achieve a particular large FSR. In add-drop filter applications, where the through-port response (relevant to express channels) is of interest, certain such Vernier designs may have excessive dispersion at suppressed resonances and thus destroy the signal modulation in some of the through port channels. Thus, filter FSR extension schemes with tolerable dispersion are of interest.
Furthermore, mechanisms for wavelength tuning of the passband of a filter, such as thermo-optic (TO), electro-optic (EO) or micro-electro-mechanical systems (MEMS)-based tuning, support a certain limited range of wavelength tunability. Moreover, the filter center wavelength is preferably to be well controlled to a fraction of the passband width. When directly tuning a resonator over a wide OWR using one of the aforementioned mechanisms, it is necessary to span the full OWR, and yet simultaneously provide the fine tuning control necessary to support the needed wavelength stability and accuracy. For example, for a 40 gigahertz (GHz)-wide filter passband on a 32 nm (4 terahertz (THz))-wide OWR containing a WDM channel spectrum, one may need better than 4 GHz control on the filter center frequency or one part in 1000 (tolerable resonance frequency error relative to the OWR). Filter architectures that permit wider tuning, more robust control of the tuning mechanism, lower power consumption, higher speed, or a combination thereof are of interest.